1. Field of the Invention
This invention relates to a drilling method, such as is used to access subsurface formations. In particular, the invention relates to methods and apparatus to facilitate the avoidance of undesirable vibration or other forms of movement in the drilling apparatus.
2. Description of the Related Art
Bores may be drilled into the earth for a number of reasons, for example to access aquifers or hydrocarbon-bearing formations. Conventionally, drilling is achieved by mounting a drill bit on the leading or distal end of an elongate support, which may take the form of a drill string. The drill bit may be driven by rotating the drill string by a surface motor, or by using a downhole motor. Drill bits take many different forms, and may be operated in conjunction with reamers or other cutting tools utilized to increase the diameter of a pre-drilled bore.
Drilling operators face increasing challenges in achieving efficient drilling of bores to access hydrocarbon-bearing formations. More readily accessible reservoirs are being exhausted, and accessing new reservoirs typically involves drilling deeper and though harder rock formations. In addition, a single drilling rig site is now often used to access formations a significant distance from the rig, such that inclined and even horizontal bores are being drilled. These changes in drilling practice require provision of longer drill strings and larger capacity surface motors.
One difficulty that operators often encounter, in drilling both shallow and longer reach bores, is that the rotating drill string may experience significant transverse and longitudinal vibration. This will result in parts of the drill string above the drill bit coming into contact with the bore wall. The resulting friction results in a drop in the torque being transmitted to the drill bit from surface, slows the rotation of the string, and may cause wear and damage to the parts of the string coming into contact with the bore wall. The contact between the string and the bore wall may also damage the bore wall. Furthermore, the presence of vibration may induce fatigue failures or other damage to parts of the string and to tools or devices incorporated in the string. Drilling operators also encounter “stick-slip”, when friction between parts of the rotating drill string and the bore wall reaches a level where a lower or distal part of the string stops rotating. The drill string motor may stall, or the upper part of the string may continue to rotate, driven by the motor on the surface, until the sum of the torque being applied by the motor and the spring energy stored in the twisted string is sufficient to overcome the frictional forces and the string begins to rotate again. In this situation, the string will initially accelerate sharply to a high rotational speed, during which period the string may experience damaging vibrations, before slowing to match the motor speed. However, the stick-slip sequence will often develop into a repeating pattern or cycle. Clearly this is inefficient, and accelerates wear and damage to the drill string elements.
Another phenomenon experienced by drill strings is whirl. In a drill string experiencing whirl, in addition to the rotation of the string about its axis, portions of the drill string flex and the axis of the string is displaced from the bore axis and follows a path around the bore axis. The path may be substantially circular and concentric with the bore axis, or may be irregular or chaotic. Whirl can be forward, that is in the same direction as the rotation of the string, backwards, that is in the opposite direction to the rotation of the string, or chaotic. It is believed whirl may be induced by a variety of conditions, primarily by a force or mass acting on a portion of the drill string to initially move the portion of the string off the axis of the bore, and then an adjacent larger diameter portion of the rotating string coming into contact with the bore wall. The string may then whirl forwards with the larger diameter portion in sliding contact with the bore wall, whirl backwards with the contact between the larger diameter portion of the rotating string and the bore wall pulling the string backwards round the bore wall, or each contact between the string and bore wall causing the string to be kicked violently away from the wall to create chaotic whirl.
Understandably, whirl is considered to be undesirable, due to the increased friction that occurs as the larger diameter elements of the drill string come into contact with the bore wall, the resulting wear on the drill string elements, damage to instruments and devices in the drill string, the potential for damage to the bore wall, the resulting shock loads on the drill string, and the stresses and strains induced in the string as the string flexes.
Drilling engineers seek to avoid the creation of excess vibration, whirl and “stick-slip” in drill strings by various methods, including: appropriate selection of drill string elements such as bits, collars and stabilizers; selection of weight applied to the string at surface; selection of torque applied to the string, and selection of string rotational speed. If a drill string begins to vibrate during a drilling operation, a drilling engineer will take various steps to seek to avoid or minimize the vibration, for example reducing the speed of rotation of the string, or the weight applied to the string may be varied. However, it is difficult to predict the combination of string and drilling conditions that will tend to induce vibration, and once such vibration has commenced it can be difficult to stop.